Question: Describe aims, objectives and importance of plant systematics (classification).
Answer: Classification is the arrangement of plants into groups having common characteristics. These groups are arranged into a system. Similar species of flowering plants are placed in a Genus, similar genera grouped into families; families with common features are arranged into orders; orders into classes and classes into divisions.
The aim of classification is to place the plants into a hierarchy of ranks or categories such as species, genera, families and so on. In addition to expressing relationship based on common features, classification serves as a filing and information retrieval system and allows easier reference to organisms comprising the filing system i.e. it provides an idea bout the sequence of evolution of plants from simple to more complex and from more primitive to more advanced types.
The criteria of the classification are the characters on which the classification is based. The characters of an organism are all the features or attributes (leaf width, stamen number, corolla length, locule number, placentation, etc) possessed by the organism that may be composed, measured, counted, described or otherwise assessed. This means that indifferences, similarities and discontinuities between plants and taxa are reflected in their character. The characters of a taxon are determined by observing or analyzing samples of individuals and recording the observations or by conducting controlled experiments.
Certain characters which are used in description, delimitation or identification are called Diagnostic characters, whereas the characters of constant nature which are used to help define a group are termed as synthetic characters. A character may be qualitative character when it refers to such things as flower colour, odour, leaf shape etc. or quantitative character when it expresses the features that can be counted or measured such as size, length, and breadth, etc. Important characters are:
(i) to provide information for construction of taxonomic system.
(ii) to supply characters for construction of keys for identification.
(iii) to furnish features useful in the description and delimitation of taxa; and
(iv) to enable the scientists to use predictive value of classification.
Morphological and anatomical characters are used for the purpose of classification. Structures are observed with the eye, hand lens or light microscope or by using scanning electron microscopy (SEM). Modern instrumentation allows comparative studies of physiological rates such as photosynthesis and analysis of chemical compounds produced by the plants. For classification purpose, there is increasing use of evidence from the fields such as cytology, biogeography, paleo-botany, phyto-chemistry, population biology, molecular biology and ulra structure.
Vegetative parts of angiosperms such as leaves stems and roots are relatively large and easy to observe but they generally provide fewer characters for classification than reproductive structures such as sepals, petals, stamens, and pistils. Vegetative features of flowering plants, especially size and shape, tend to be more influenced by environmental features than are reproductive features. As a result certain vegetative characters are less reliable and less useful than reproductive characters in classification of plants.
Union of sepals, petals, stamens and corpels; epipetalous condition, placenlation, hypogyny, epigyny and perigyny; number of cotyledons, presence and absence of endosperms, structure of fruit and seed are common reproductive characters used for classification. Phylotais, venation and presence of absence of stipules, are vegetative characters used for classification.